The present invention relates to optical fiber. More particularly, the present invention relates to metal-coated optical fiber, and techniques for manufacturing same.
Optical fiber is typically constructed having a polymer coating, but some applications necessitate the use of metal-coated optical fiber. For example, distributed fiber sensing technology for temperature, acoustic vibration and strain have become popular in oil and gas well monitoring. The well temperature in oil sands or super heavy oil reservoir sometimes becomes more than 300 deg. C. because thermal extraction enhancement is applied frequently. Current polymer coated fiber does not keep its original mechanical properties against such high temperatures. Thus, metal coated fiber is applied for high temperature environment instead of polymer coated fiber.
Metal coated fibers such as aluminum, copper and gold are commercially available. But all of these fibers have thick metal layers more than 20 micron because dipping methods are applied for their manufacture. In particular, bare fiber is dipped into molten metal during passing coating die filled with molten metal and then frozen on the fiber surface. One disadvantage of these fibers is larger attenuation because thicker coating thickness of around 20-30 micron and thermal contraction by freezing leads to additional loss. For example, a typical loss of copper coated fiber with 125 micron of glass diameter and 20 micron thickness of copper is around 10 dB/km at 1310 nm.
As an alternative manufacturing method of metal coated fiber, it was reported that a low loss metal coated fiber was made by a plating method. (International Wire & Cable Symposium Proceedings 1991, pages 167-171.) The attenuation of the reported fiber with 125 micron glass and 2.5 micron of nickel layer is 0.7 dB/km at 1300 nm. The structure of metal coated fiber made by plating is described in U.S. Pat. No. 5,093,880, which is incorporated herein by reference for all purposes. But long metal coated fiber made by plating is not yet commercialized due to the difficulty of handling bare fiber. A method of manufacturing metal coated fiber by plating without degrading mechanical reliability is disclosed in application no. PCT/US2014/028151 (published on Sep. 25, 2014 as WO 2014/152896). U.S. Pat. No. 5,093,880 and application no. PCT/US2014/028151 are both incorporated fully herein by reference for all purposes.
As for the performance of temperature resistance, only loss performance under high temperature was previously described. The bending performance after heat treatment or ductility against high temperature environment was not reported. However, retaining ductility after heat treatment is an important mechanical performance characteristic for downhole cable. In this regard, sensing cable is installed into well repeatedly for logging. Thus, repeated mechanical movement is applied to the sensing cable. And even sensing cable installed permanently has been affected by mechanical vibration and other mechanical movement that occurs during well production and operation. Thus, keeping ductility after heat treatment as well as loss performance are important criteria in the downhole application.
The present invention recognizes the foregoing considerations, and others, of the prior art.